Several series of new dimeric analogs of enkephalin penta-, tetra- and tri-peptides have been synthesized, purified, and characterized by several physical chemical techniques, binding assays, bioassays, and in vitro bioassays. The new analogs show extremely high potency and specificity for the "delta" opiate receptor, and are useful in the characterization of opiate receptors, autoratiographic and pharmacological applications. Binding kinetics are compatible with the hypothesis that the ligands bind simultaneously to two receptors. Effects of ions (Mn2+, Na+) and GTP on binding of monomers and dimer have been compared. The ability of the dimers to induce desensitizaion and tolerance in NG108-15 cells has been characterized, and appears to differ qualitatively from that of the monomers. Additional new compounds include alkylenkephalin amides and a mono-N-acetyl enkephalin. The latter compound provides a novel probe of the delta receptor.